This study attempts to pharmacologically characterize axonal endings in the dorsal horn of the medulla (DHM). The methods employed include light and electron microscopic autoradiographic localization of labeled axonal endings in the DHM following application of tritiated (3H) neurotransmitters onto the DHM and immunohistochemical localization of specific populations of axonal endings. The neurotransmitters being investigated include serotonin, norepinephrine, enkephalin and substance P. The purpose of these studies is to locate and morphologically characterize pharmacologically distinct axonal endings in the DHM which might be involved in trigeminal pain pathways.